In the medical arts there are known a variety of systems for assisted ventilation of patients and for administration of anesthetic gas. With prior art ventilation apparatus, a patient typically is ventilated by an electro-mechanical ventilator which delivers oxygen enriched air to the patient on a predetermined, programmed basis. Examples of prior ventilation techniques include CPAP, assisted or controlled ventilation and intermittent mandatory ventilation, among others. With many prior cyclically operating ventilation systems, the patient's lungs are ventilated by cycling airway pressure between ambient atmospheric pressure and some higher ventilation pressure. During the high pressure phase of the cycle, the lungs are inflated with the breathing gas mixture supplied by the system. During the ambient pressure phase, the lungs deflate as the patient spontaneously exhales the gas into the atmosphere or other suitable exhaust facility. With CPAP therapy, a continuous positive pressure is superimposed on the ambient pressure phase, as is well known.
Prior ventilation system tubing circuits commonly include a gas supply side and an exhaust or exhalation side, and patient airway pressure is controlled by an exhalation valve which is installed in-line with the exhalation side of the tubing circuit. Prior exhalation valves typically have been controlled by a control pressure generated by the ventilator apparatus independently of the gas supply pressure. The control pressure is effective, when the ventilator begins an inhalation cycle, to close the exhalation valve and allow the pressure in the circuit to be elevated to the desired airway pressure. The patient thus experiences assisted inhalation. When the ventilator stops positive pressure gas delivery, the exhalation valve control pressure is reduced to permit elevated patient airway pressure to open the exhalation valve and the patient thus exhales through the exhalation valve.
Another known ventilatory therapy controller is the PEEP valve, which may be used in conjunction with the exhalation valve in order to maintain exhalation pressure at a pressure higher than ambient and thereby keep the lungs under positive pressure throughout the exhalation phase of the cycle. Positive expiratory pressure has also been obtained by controlling the exhalation valve itself to provide positive expiratory pressure.
Among the more common medical gas delivery circuits are the conventional circle anesthesia system and partial rebreathing anesthesia or resuscitation systems. Among the prior art patents known to the applicant which pertain to valves for respiration and anesthesia apparatus generally, are the following: U.S. Pat. No. 3,017,881 shows a system for administering anesthesia to a patient wherein the patient inhales a gas mixture including an anesthetic and exhales through a conduit leading to an exhaust valve. Others include U.S. Pat. Nos. 4,502,502; 4,044,793; 4,037,595; 3,800,793; 3,721,239; and 2,868,198, and French Patent No. 2,003,192.
Although the conventional systems have generally served their intended purposes, practitioners of the art continue to seek improved systems offering enhanced economy, reliability and ease of maintenance and operation. For example, in common usage, the external valve and tubing circuitry of a ventilator circuit often may have to be changed on a daily or perhaps semi-daily basis in order to minimize the possibility of bacterial growth within the system which could infect or reinfect the patient. Frequent replacement of conventional circuits is rather expensive as the circuitry often comprises substantial lengths of tubing and multiple valves which must be either cleaned up for reuse or discarded and replaced with new valve and tubing circuitry.
Another shortcoming of many prior ventilation systems is that, due to the necessary valving arrangements they include relatively long lengths of tubing in separate air supply and exhaust legs. This compounds the difficulties that are inherent in determining the patient's tidal volume and lung-thorax compliance, determinations which must be accurate to ensure proper adjustment and programming of the ventilator to provide the most thereputically beneficial ventilatory assistance. Excess tubing lengths introduce both an enlarged working volume of air in the tubing circuitry, and an increment of system compliance, similar to lung compliance, as the resiliently flexible tubing is cyclically distended and relaxed in response to the pressure variations imposed during the ventilation cycle. These variables are superimposed on the parameters of lung mechanics of the patient, and their impact must be fully appreciated and taken into account before the ventilation cycle can be properly programmed.
Another conventional anesthesia system includes a manual breathing bag which the physician squeezes to ventilate the patient. Manual squeezing of the breathing bag increases the system supply pressure sufficiently to close the exhaust valve and thereby force the system gas mixture into the patient's lungs. When the physician releases the breathing bag, the exhaust valve is permitted to open again. Such systems may often includes a pressure relief valve connected to the anesthesia scavenging system, that may be manually adjusted to open only if the pressure in the system exceeds a predetermined valve setting during system operation. For such systems, the physician may manually ventilate the patient by first adjusting the relief valve to increase its pressure relief set point so that sufficient pressure can be applied by squeezing the breathing bag for the desired ventilation without opening the relief valve. The relief valve must be readjusted after patient ventilation.
Even in systems which do not accommodate such manual ventilation, but instead merely provide a gas mixture at a constant volume flow rate for unassisted breathing by the patient, a relief valve is provided to relieve pressure excursions resulting from discontinuities between the gas supply rate and the patient's gas consumption and breathing rate. Improper adjustment of the relief valve can result in system supply pressure excursions of dangerous proportions.